Since the invention by Colin Sullivan of the use of nasal Continuous Positive Airway Pressure (nasal CPAP) to treat “snoring sickness” (see U.S. Pat. No. 4,944,310), there have been a number of advances directed towards improving the noise and comfort of therapy. In nasal CPAP therapy, a supply of air at positive pressure is delivered to the entrance of a patient's airways via an air delivery conduit and some form of patient interface, such as a mask. The early masks were custom made for each patient and glued on each night. A typical mask comprises: (i) a frame which defines a nose-receiving mask cavity; (ii) a seal-forming face-contacting cushion which in use is positioned between the frame and the patient's face; and (iii) a vent to atmosphere which amongst other things allows exhaled CO2 to vent to atmosphere, thus reducing CO2 rebreathing.
It is generally desirable for the treatment system (including the source of pressurized air and the patient interface) to be as quiet as possible so as not to disturb sleep.
The supply of air at positive pressure may be provided by a blower, sometimes referred to as a flow generator. Such devices typically include an electric motor and impeller housed in a volute. Spinning the motor (and thus the impeller) generates a flow of air. When the flow is attached to an air circuit, a pressure is created due to the impedance of the circuit. Spinning the motor faster generates a supply of air at higher pressure, but also more noise. As a fluid such as air flows through a pipe or conduit it loses pressure. Bends and curves in the pipe affect the amount of pressure loss. See Perry's Chemical Engineers Handbook 6th Edition, McGrawHill, 1984, Section 5, Fluid and Particle Mechanics. The greater the pressure drop in each component (i.e. the higher the impedance) of the air circuit (for example along the air delivery conduit) the harder the blower must work in order to provide sufficient pressure in the patient interface. The harder the blower has to work, the greater noise it will generate. Thus generally it is important to design components in the air path to have a low impedance.
A further reason for minimizing the impedance of components in the air path is to minimize pressure swings as the pressure fluctuates within the mask due to the patient breathing. A higher entry impedance at the mask will lead to a higher pressure difference between inspiration and expiration, which may lead to patient discomfort and additional cyclic noise.
The process of air venting from the mask creates noise. Since patients must wear their mask all night while sleeping, there is a need for the vent to be quiet. Some quiet vents are described in U.S. Pat. No. 6,561,190 (Kwok et al.) and U.S. Pat. No. 6,561,191 (Kwok et al.). The contents of these two patents are hereby expressly incorporated by cross-reference.
While in some mask designs—such as the ResMed MIRAGE mask—the air delivery conduit is fixed in position in relation to the frame, other masks—such as the ResMed ULTRA MIRAGE mask—include a swivel elbow. The swivel elbow enables the air delivery conduit to rotate with respect to the mask. This enables a patient to place the air delivery conduit in a preferred position such as over the head or on the left or right sides. Absent a swivel, inadvertent movement of the air delivery conduit can disrupt the seal and thus therapy.
In designing hard parts for patient interfaces, such as a mask frame and elbow constructed from polycarbonate or similar materials, regard must be had to how the part will be molded. For ease of manufacture, the tool from which a component is manufactured generally has two parts that form the shape of the component. Once the component has been formed, the tool is opened by withdrawing one part along a “line of draw” that is of constant radius (including a straight line). Parts must be designed within the constraints of what is manufacturable.
Some swivel elbows, such as the one used in ResMed's ULTRA MIRAGE mask, incorporate a vent. See U.S. Pat. No. 6,691,707 (Gunaratnam et al.). Incorporating a vent in a swivel elbow can allow the patient some control over the direction in which air is vented. Thus the vented air may be directed away from the patient or anyone sleeping close by. Incorporation of a vent in an elbow can simplify molding of the mask frame.
Vent flow rate, and hence vent CO2 flow rate is a function of the pressure differential between the mask interior and ambient pressure. The higher the differential, the higher the flow rate. With a fixed vent, whether adequate CO2 washout occurs is defined by what happens at the lowest operating mask pressure, typically 4 cmH2O. The flow rate is also a function of vent geometry.
In some prior art vents incorporated in elbows, air entering the elbow from a blower can short-circuit the mask and pass straight out the vent.
Another known swivel elbow which includes a vent is described in International Patent Application PCT/AU2003/001162 (published as WO 2004/022147) to Drew et al., the contents of which are hereby expressly incorporated by cross-reference. This elbow includes a baffle in the elbow as described in the '1162 PCT application. A commercial version of this elbow is found in ResMed's ACTIVA mask system.
A potential problem with including a baffle in the elbow is that while it may assist with CO2 washout, it may impede flow from the blower. Increased impedance from a baffle may require a blower to work harder to generate enough pressure and thus result in increased noise. A poorly designed baffle and corresponding vent may be unnecessarily noisy. A possible way of avoiding increased impedance is to make the elbow larger overall, however this is undesirable for other reasons such as aesthetics.